Conventionally, in the image forming apparatus of the electrophotographic type, an image is formed by an electrophotographic process including charging, exposure, development and transfer. Of these, in a charging step, an electrophotographic photosensitive member (photosensitive member) has been electrically charged uniformly to a potential of a predetermined polarity by a corona charger provided close to the photosensitive member.
The corona charger has a constitution in which a discharge wire (discharging electrode) is extended between blocks as holding members provided at end portions of a shield (shield case) which is a supporting casing having an open (one) surface. The shield is representatively constituted by a stainless steel (hereinafter also referred to as “SUS”). Further, the discharge wire is constituted by a wire material of tungsten or the like. In the case where a surface-to-be-charged of a member-to-be-charged such as an image bearing member is electrically charged using the corona charger, the corona charger is caused to oppose the member-to-be-charged in a state in which an opening of the shield is brought near to the surface-to-be-charged, and a discharge current is supplied to the discharge wire to generate corona discharge, so that electric charges are imparted to the surface-to-be-charged.
A charge potential of the surface-to-be-charged is controlled by providing a grid (grid electrode) between the discharge wire and the image bearing member and by adjusting an amount of the electric charges imparted to the image bearing member by a grid bias applied to the grid. As the grid, principally, a wire-like grid formed of the same material as the discharge wire or of SUS, and a plate-like grid in which many holes are formed by etching or the like.
Incidentally, in the image forming apparatus using the electrophotographic type, the grid of the corona charger for electrically charging the surface of the image bearing member is contaminated with a scattered matter such as an electric discharge product, toner, an external additive or paper powder. Further, in the case where particularly the plate-like grid is used, the holes are clogged with the scattered matter deposited on the grid during corona discharge, whereby a rectifying effect is impaired and therefore potential spots are generated on the image bearing member in some cases. In order to maintain high-quality image formation, it is desired that these discharge spots are remedied and an image defect is prevented.
As regards the above-described scattered matter, it has been proposed that a cleaning member is provided in the corona charger and the scattered matter is removed by the cleaning member. Japanese Laid-Open Patent Application 2005-338797 discloses that the scattered matter deposited on the plate-like grid is removed by the cleaning member.